diff --git a/js/modules/types/message.js b/js/modules/types/message.js
index 8a5081f90..5df444c8f 100644
--- a/js/modules/types/message.js
+++ b/js/modules/types/message.js
@@ -13,9 +13,11 @@ const PRIVATE = 'private';
 // Version 0
 //   - Schema initialized
 // Version 1
-//   - Attachments: Auto-orient JPEG attachments using EXIF `Orientation` data
+//   - Attachments: Auto-orient JPEG attachments using EXIF `Orientation` data.
 // Version 2
-//   - Attachments: Sanitize Unicode order override characters
+//   - Attachments: Sanitize Unicode order override characters.
+// Version 3
+//   - Attachments: Write attachment data to disk and store relative path to it.
 const INITIAL_SCHEMA_VERSION = 0;
 
 // Increment this version number every time we add a message schema upgrade
@@ -23,7 +25,7 @@ const INITIAL_SCHEMA_VERSION = 0;
 // add more upgrade steps, we could design a pipeline that does this
 // incrementally, e.g. from version 0 / unknown -> 1, 1 --> 2, etc., similar to
 // how we do database migrations:
-exports.CURRENT_SCHEMA_VERSION = 2;
+exports.CURRENT_SCHEMA_VERSION = 3;
 
 
 // Public API
@@ -163,5 +165,8 @@ const toVersion3 = exports._withSchemaVersion(
 );
 
 // UpgradeStep
-exports.upgradeSchema = async (message, context) =>
-  toVersion2(await toVersion1(await toVersion0(message)));
+exports.upgradeSchema = async (message, { writeAttachmentData } = {}) =>
+  toVersion3(
+    await toVersion2(await toVersion1(await toVersion0(message))),
+    { writeAttachmentData }
+  );
diff --git a/test/modules/types/message_test.js b/test/modules/types/message_test.js
index e36538f88..ddd8d0c33 100644
--- a/test/modules/types/message_test.js
+++ b/test/modules/types/message_test.js
@@ -1,3 +1,4 @@
+const stringToArrayBuffer = require('string-to-arraybuffer');
 const { assert } = require('chai');
 
 const Message = require('../../../js/modules/types/message');
@@ -66,7 +67,7 @@ describe('Message', () => {
       const input = {
         attachments: [{
           contentType: 'application/json',
-          data: null,
+          data: stringToArrayBuffer('It’s easy if you try'),
           fileName: 'test\u202Dfig.exe',
           size: 1111,
         }],
@@ -75,14 +76,21 @@ describe('Message', () => {
       const expected = {
         attachments: [{
           contentType: 'application/json',
-          data: null,
+          path: 'abc/abcdefg',
           fileName: 'test\uFFFDfig.exe',
           size: 1111,
         }],
         schemaVersion: Message.CURRENT_SCHEMA_VERSION,
       };
 
-      const actual = await Message.upgradeSchema(input);
+      const expectedAttachmentData = stringToArrayBuffer('It’s easy if you try');
+      const context = {
+        writeAttachmentData: async (attachmentData) => {
+          assert.deepEqual(attachmentData, expectedAttachmentData);
+          return 'abc/abcdefg';
+        },
+      };
+      const actual = await Message.upgradeSchema(input, context);
       assert.deepEqual(actual, expected);
     });